fix: avoid rust-analyzer snake case warning

It seems that rust-analyzer needs to operate over the expanded text of the proc macro (in order to e.g. support completion in the function body, see #11014 for way more details), so it "sees" the non-snake-case name emitted by riscv-rt's `entry` here.

Without this change, rust-analyzer will show a "weak warning" on invocations of `#[entry]` with the text:

```
Function `__risc_v_rt__main` should have snake_case name, e.g. `__risc_v_rt_main`
```